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THURSDAY, June 19 (HealthDayNews) -- Just in case you thought female sexuality couldn't get any more complicated.

Researchers, armed with pornographic movies, say they have discovered that women, regardless of their sexual orientation, tend to be aroused by scenes featuring heterosexuals, lesbians or gay men.

The world of sexual arousal is simpler for men. In general, straight men only get turned on while watching women have sex, either with men or with each other, according to the study's findings.

"Sexual orientation for women seems to be a very different thing than it is for men," having to do with more than just sexual arousal, says study co-author Michael Bailey, chairman of the psychology department at Northwestern University. Otherwise, many of the women in the study would have been bisexual, he says.

The research will appear in an upcoming issue of Psychological Science, Bailey says.

Bailey and his colleagues decided to launch a general study of sexual arousal and orientation because most research on the topic has focused on men. During the past several years, they invited 121 subjects -- 69 men and 52 women -- to watch X-rated videos as researchers monitored their sexual response. All the subjects reported being exclusively or near-exclusively heterosexual or homosexual; none said they were bisexual.

To gauge arousal, the men connected themselves to a device that measures the stiffening of the penis. Not surprisingly, it was a bit more challenging to figure out whether women were stimulated. The researchers used a plastic tube device that shines a light into the vagina and measures the level of reflected light. The vagina becomes darker during arousal, Bailey explains.

As the researchers monitored the arousal-measurement devices from another room, the men and women watched two-minute snippets of commercial pornography depicting explicit scenes of sexual activity. The videos showed sex between men, between women, and between men and women.

The results showed a stark difference between male and female sexual responses. "Women, no matter what their sexual orientation was, tended to respond to both male and female erotica, and even male-male erotica," Bailey says. "Straight men responded to female erotica, and gay men responded to male erotica."

Nearly all the men responded more to videos featuring their preferred gender -- men or women. But only 63 percent of women did.

Judging by their arousal levels, heterosexual women were about equally turned on by male-male, female-female and male-female sex scenes. However, heterosexual men were much more excited by lesbian sex scenes than those featuring men and women.

So were the women in the study bisexual? Only in terms of their sexual arousal, not in the decisions they make about their sexual relationships, Bailey says. "Women's sexual arousal pattern is not all that relevant to their sexual orientation," he says.

Lisa Diamond, an assistant professor of psychology at the University of Utah, says Bailey's study reflects the findings of previous research. Sexual orientation is "a lot more of a complicated phenomenon than it seems on the surface," influenced by various emotional and physical factors, especially in women, she says.

"With women, the experience of sexual attraction doesn't revolve around the gender of the partner as it does around other things," Diamond says.

More information

Learn more about sexual orientation from the American Psychological Association. The APA also offers insights into sexual arousal.